In industry, air and pressurized (compressed) air are widely employed in operating equipment and machinery in manufacturing, product fabrication, and in countless other applications and scenarios. Air and compressed air in particular, is often laden with moisture which negatively impacts on the performance and life expectancy of equipment, machinery, applications and processes, ultimately resulting in costly equipment failure, downtime and befouled product.
One desirable method employed in removing moisture from air and other gases has been through the use of a drying agent or desiccant (adsorbent). Desiccant dryers have been one of the premiere means for successfully removing substantially all moisture from air for industrial applications thereby reducing equipment failures and improving product quality. Desiccant air dryers typically are comprised of a pressure vessel filled with the desiccant material, piping and valves as means for controlling air flow throughput and means for purging moisture from the drying agent. Generally, these systems can have other useful features, such as filters to remove oil and dirt, screens to contain the desiccant within the vessel, electronic controls, liquid draining devices, and so on. In addition, many configurations of desiccant air dryers are constructed of dual components, so that the apparatus may be on-line using one set of components while the second set is regenerating itself of captured (adsorbed) moisture. Such systems are known as "Twin Tower" dryers.
Because desiccant loses its ability to effectively remove moisture after a period of use, desiccant filled gas dryers are required to operate in a regenerative mode with a desorption half-cycle to remove water from the drying agent to reactivate it for another adsorption half-cycle. This regeneration process represents a significant amount of unproductive downtime and direct costs in operation of heaters and/or blowers, or in the case of dry purge methods, to extract the collected moisture from the desiccant material. Regeneration of the desiccant and purging moisture from the dryer can take several hours to complete, and represents typically fifty percent of the duty cycle of the dryer, e.g., 4 hours on-line for removing water from an air stream, and 4 hours off-line for regenerating, cooling and repressurizing, depending of the capacity/size of the system and air volume.
As previously stated, one aspect of the problem of desiccant regeneration stems from the high energy requirements in the form of heat and/or electrical power needed to remove moisture and dry the desiccant. Heretofore, in the regeneration phase it has been the practice to inject heated purging air into the pressure vessel at one end and force it through the moisture laden desiccant bed and out the other end of the vessel. Moisture adsorbed by the desiccant is routinely more concentrated at the first end or air entry end of the vessel and is less concentrated in the region of the second end or air exit end of the vessel. In an "up-flow dryer" the bottom or first end of a vessel holding the desiccant bed is the air entry end during the air drying phase of a cycle (adsorption half-cycle) while the second end or top end of the pressure vessel is the air entry end during the regeneration phase of a cycle (desorption half-cycle) or moisture purge mode in removing water from the adsorbent. Conversely, in a "down-flow dryer" the top or first end is the air entry end during the air drying mode of the cycle while the bottom or second end is the air entry end during the regeneration mode of the cycle.
Irrespective of the type desiccant air dryer, either up-flow dryer or down-flow dryer, the practice of injecting hot air through a single port at one end of the vessel in the regeneration mode results in an undesirable temperature imbalance when purging moisture from the desiccant. Conventional regeneration processes typically overheat the vessel output end because of the prolonged time required to sufficiently reach a proper temperature at the vessel input at the opposite end, where most of the moisture resides. That is, the time required for the flow of heated air to purge or desorb the stratified portions of the desiccant most heavily saturated with moisture at the vessel input end results in excessive heating of the vessel and desiccant, wasting energy and time to allow these overly heated areas to sufficiently cool before returning the unit to drying air or other gases.
To illustrate further, in the desorption half-cycle (regeneration mode) it has been found that the temperature can range from about 350.degree. F. to about 650.degree. F. or more, at the heated air entry end while only 200.degree. F. at the air exit end of the vessel during regeneration. This imbalance leads to wasted energy and higher operating costs through higher temperatures in vessel heating and longer operation of blower motors for the period of time required to purge substantially all the moisture from throughout the bed. Other consequences of this temperature imbalance include overly (or unevenly) dried desiccant at one end of the bed; protracted downtimes due to longer cool down periods before the system can be returned to service (adsorption cycle); and dew point "bump" or spikes where moisture laden air remains in the desiccant bed due to a temperature hot spot. Such moist air, if forced out of the dryer as dry air can result in down stream technical problems in the application of the use of dried air
Accordingly, there is a need for an improved heat regenerated desiccant gas dryer and method of use which will consume less energy and use it more efficiently, permit shorter turn around times in regenerating the desiccant bed in removing water during the desorption half-cycle, avoiding temperature imbalances and over heating the apparatus, and elimination of hot spots and spikes or dew point bumps.